Not illegal, just requires a tax stamp and some paperwork, unless WA won't allow SBRs.

There's the concept of constructive possession.

The idea is that you can have a legal rifle and be OK. You can have a short rifle barrel and be OK (as long as you don't have a rifle to put it on). If you have the rifle and the short barrel, then you are in trouble because you could easily make a short-barreled rifle and therefore are in constructive possession of a non-existent short-barreled rifle.

If it's just a barrel for something like a bolt gun, you could argue that you don't have the tools to swap the barrel and hope for the best. If it's an AR upper that can be swapped without tools, then you're in trouble.

On the other hand, if you have a regular barrel AR, an AR pistol receiver, and a short AR upper, you can argue you actually have a rifle and a pistol, even though you could easily make an SBR, you have a legitimate reason to have the short upper.

It gets tricky when you get into this territory, so make sure you know the laws better than from internet advise or you can end up in jail.